1. Field of the Invention
The present invention relates to a thermal head manufacturing method, a thermal head manufactured by the method, and a printer equipped with the thermal head.
2. Description of the Related Art
As a thermal head to be used in a thermal printer, there is conventionally known a thermal head having a cavity portion which is formed in a region opposed to a heat generating resistor of a substrate and which functions as a heat-insulating layer having a low thermal conductivity. Due to this configuration, this conventional thermal head is designed to reduce an amount of heat to be transferred from the heat generating resistor to the substrate side, thereby improving heat generation efficiency and reducing power consumption.
In one example of this type of conventional thermal head, a groove or a through hole is formed in an electrically insulating substrate made of alumina, or the like. In this conventional thermal head, a low softening point material covers the groove or the through hole, a high softening point material covers the low softening point material, and baking is performed. In this way, the high softening point material becomes a glaze layer, and a cavity portion is formed between the substrate and the glaze layer by softening the low softening point material and causing the low softening point material to move into the groove or the through hole of the substrate.
In another conventional thermal head, a concave portion is formed in one of a substrate and a heat storage layer bonded thereto, and the concave portion is closed by the substrate and the heat storage layer so as to form a cavity portion therebetween. Alternatively, an adhesive layer to which the substrate and the heat storage layer are bonded is used, and there is provided a region in which the adhesive layer is not formed between the substrate and the heat storage layer so that the region serves as a cavity portion.
However, the above-mentioned conventional thermal heads have various problems. Specifically, in the former thermal head, the high softening point material is made of a glass paste having a softening point of 860° C., and hence, when baking is performed at 1,000 to 1,200° C., the high softening point material needs to be prevented from moving into the groove or the through hole together with the low softening point material, and a high heat-resistant film of SiNx, Sic, and the like is required to be provided as a partition on the low softening point material. Therefore, a sputtering step is necessary between paste printing of the low softening point material and paste printing of the high softening point material, and there is a problem of increased manufacturing cost and increased manufacturing time. Further, in order to prevent the groove or the through hole of the substrate from being filled when the low softening point material is printed, it is required to precisely control the temperature of the paste, the solvent concentration, the usage time, and the like so as to always maintain the viscosity of the glass paste to be constant. Therefore, even in this regard, there is a problem of increased manufacturing cost.
In the latter thermal head, when alumina for the substrate and glass for the heat storage layer are bonded to each other, the adhesive layer is necessary because the heat storage layer cannot be directly bonded to the substrate while maintaining the shape thereof. However, there is a problem in that a polymeric material becomes soft or deteriorated because of the heat generated in printing, the bonding force between the substrate and the heat storage layer is reduced at a high thermal expansion coefficient, and reliability is reduced. In the case where the substrate and the heat storage layer are bonded to each other using a glass paste for the adhesive layer, when the bonding temperature is set to be higher than a softening point of the glass paste, there is a problem in that the adhesive layer is liquefied to lose the shape thereof, and hence the cavity portion having a desired shape is not formed, and the expected thermal efficiency is not obtained. On the other hand, when the bonding temperature is set to be lower than the softening point of the glass paste, there is a problem in that a part where binder has evaporated becomes a cavity, glass particles are bonded to each other only in a point contact state, that is, a spongy state is produced, and the adhesive layer is crushed because of the pressurizing force of the platen roller. In the case where the glass paste is used to form the cavity portion, the shape other than the cavity portion needs to be subjected to printing and control. Therefore, in consideration of flowage of the paste and the like, the viscosity of the glass paste needs to be controlled precisely so as to be always constant, and there is a problem of increased manufacturing cost. Further, in the case where the glass is used for the substrate, heat is stored in the glass substrate, and tailing occurs when printing is performed continuously. Therefore, there is a problem in that the print quality is degraded and the printing speed is not fast.